I upgraded my Xfinity service back in July to 200 Mbps, however I'm averaging only 40 Mbps. I don't have TV, only internet. What could be the issue?
Try calling 1-800-comcast and ask a rep to completely remove the modem from your account and to "re-provision" it from scratch. Also ask them to confirm that the provisioning and account data in their database is correct and complete and that the correct configuration / bootfile for your subscribed to speed tier is being pushed to the modem.
I used to pay for 100 and get 20-30. I upgraded to 300mbps download, bought a dual and router 2.4 and 5ghz but the real issue was the coax connection. I took off the coax connector and plugged it back on making sure the needle of the connection was in the hole and securely fastened. Did the same to the router. It was just the connector to the wire leading outside the house had gotten jostled and loosened, I am now pulling 320mbps down. Still a little slow on the 2.4ghz frequency but that may be the router, super fast on 5ghz frequency. I would adjust your coax connection.